Stepping motor drive circuit providing collapsing field energy storage

ABSTRACT

A drive circuit for supplying current to individual coils of a stepping motor for the duration of a signal pulse, the drive circuit being characterized by a high rate of current decrease in the coil at the end of the signal pulse and efficient use of power sources. The drive circuit has a bridge configuration with the stepping motor coil connected across one bridge diagonal. Connected across the other bridge diagonal is an energizing potential such as a DC power source. Two opposite sides of the bridge have diodes connected therein, the diodes being oriented so as to be back biased by the energizing potential. The remaining two opposite sides of the bridge contain switching transistors, oriented so as to be forward biased by the energizing potential. Switch control means, responsive to the signal pulses, are provided to turn on the switching transistors at the beginning of a signal pulse to energize the coil, and to turn off the transistors at the end of a signal pulse so that the coil will discharge through the diodes back into the energizing potential for storage of the energy of discharge from the coil.

United States Patent 1 McDonald William C.

[451 July 24,1973

[75] Inventor: McDonald William C., Belmont,

Mass.

[73] Assignee: Cambridge Thermionic Corporation, Cambridge, Mass.

[22] Filed: May 3, 1971 [21] Appl. No.: 139,722

[52] U.S. Cl 318/138, 318/696, 318/439, 321/45 ER [51] Int. Cl. H02p9/04 [58] Field of Search 318/696, 685, 135, 318/437, 341; 321/45 ER[56] References Cited UNITED STATES PATENTS 3,560,821 2/1971 Beling321/45 ER 3,560,817 2/1971 Amato.... 318/138 3,525,100 8/1970 Duff321/45 X 3,530,347 9/1970 Newell.... 318/696 3,444,447 5/1969 Newell318/696 3,486,096 12/1969 Van Cleave 318/696 3,560,818 2/1971 Amato318/45 3,609,511 9/1971 Risberg 321/45 ER 3,500,170 3/1970 Charrin e al321/45 3,569,819 3/1971 Martzloff 321/45 Primary Examiner-G. R. SimmonsAttorney-Sewall P. Bronstein, John D. Woodberry, Robert T. Gammons,Donald Brown, Robert L. Goldberg and Robert F. OConnell [57] ABSTRACT Adrive circuit for supplying current to individual coils of a steppingmotor for the duration of a signal pulse, the drive circuit beingcharacterized by a high rate of current decrease in the coil at the endof the signal pulse and efficient use of power sources. The drivecircuit has a bridge configuration with the stepping motor coilconnected across one bridge diagonal. Connected across the other bridgediagonal is an energizing potential such as a DC power source. Twoopposite sides of the bridge have diodes connected therein, the diodesbeing oriented so as to be back biased by the energizing potential. Theremaining two opposite sides of the bridge contain switchingtransistors, oriented so as to be forward biased by the energizingpotential. Switch control means, responsive to the signal pulses, areprovided to turn on the switching transistors at the beginning of asignal pulse to energize the coil, and to turn off the transistors atthe end of a signal pulse so that the coil will discharge through thediodes back into the energizing potential for storage of the energy ofdischarge from the coil.

4 Claims, 4 Drawing Figures LEVEL SHIFTING CIRCUIT 1 l8 PULSE 4 SOURCE f26 PAIENIED 3.748.554

SHEET 1 (IF 2 1o 12 1 S'GNAL n COUNTER PULSE GEN. I 2 n I 1 2"! nt PHASEPHASE PHASE DRIVE DRIVE DRIVE Ll L2 Ln A I I N l )\l6o C 1Cf SWITCHPULSE $2 CONTROL7 SOURCE MEANS FIGZ PATENFEU JUL 2 75 SHEEI 2 BF 2 LEVELCIRCUIT SHIFTING FIG?) SHIFTING CIRCUIT 27 LEVEL STEPPING MOTOR DRIVECIRCUIT PROVIDING COLLAPSING FIELD ENERGY STORAGE BACKGROUND OF THEINVENTION The field of the present invention relates to circuits used toenergize stepping motor coils in response to instructions in the form ofsignal pulses.

Stepping motors, as exemplified by the stepping motor disclosed in theco-pending application of Frank N. Lyman, Jr., Ser. No. 144,494, filedMay 18, 1971 have a number of phases, each comprising a set of rotorpoles, a set of stator poles alignable with the rotor poles, and a coilenergizable to bring the rotor and stator poles into alignment throughmagnetic interaction. The phases have their rotor or stator polessuccessively offset by an angle equal to 360/NM where N equals thenumber of phases and M equals the number of rotor or stator poles ineach phase. The phases are energized sequentially with a phase lag equalto their angular offset, thereby bringing successive sets of poles intoalignment and causing rotation of the shaft to which the rotors areaffixed.

As one of the advantages of stepping motors is their ability to provideprecise angular control, useful for numerical or pulse programmedcontrol of machinery, for example, it is important that the energizationof the stepping motor coils take place within precise time limits asdetermined by a signal pulse. In the allotted period, current must besupplied to the coil and then terminated. To obtain high torque and highspeeds, it is advantageous to provide a high rate of increase of coilcurrent at the beginning of a signal pulse without causing unnecessaryenergy dissipation during the remainder of the pulse, as is more fullyset forth in my copending application Ser. No. 139,721, filed May 3,1971, under the title Stepping Motor Drive Circuit with SwitchControlled Coil Energization.

A related problem of coil energization occurs at the end of theenergization period when coil current is to be rapidly reduced to zeroso that there will-be no attraction of rotor and stator poles in adirection opposite to that intended. To reduce current to zero quickly,the energy of the collapsing field in the coil must be rapidly removed.Heretofore, this has been accomplished by transferring the stored coilenergy to an energy dissipative device such as a resistor or Zener diodewhere it does no productive work and sometimes creates a coolingproblem.

SUMMARY OF THE INVENTION Objects of the present invention are to providea stepping motor drive circuit acting in response to asignal pulse toenergize a stepping motor coil, which is capable of channelingcollapsing field energy in a way enabling it tobe reused, which issimple and economical to construct and reliable in use, and which iscompatible with a variety of coil energizing methods.

The stepping motor drive circuit according to the invention suppliescurrent to a stepping motor coil in response to a signal pulse andcomprises, in bridge configuration, a source of energizing potential,such as a rectified DC voltage source using a filter capacitor asstorage element, connected across one bridge diagonal. Connected acrossthe other bridge diagonal is the stepping motor coil. In two oppositesides of the bridge, diode means are provided, oriented so as to be backhi ased by the source of energizing potential. In the remaining twoopposite bridge sides, switch means are connected, the switch meanspreferably comprising transistors oriented to be forward biased by thesource of energizing potential. Responsive to the signal pulses, andconnected to the transistor switch means, are switch control meanscausing the transistor switch means to conduct at the beginning of asignal pulse, thereby to energize the coil, and causing the transistorswitch means to become non-conductive at the end of a signal pulse,whereby the collapsing field of the coil will direct current through thediode means back into the source of energizing potential where it can bestored, as in a filter capacitor, and made available for subsequent use.

These and other objects and novel aspects of the invention will beapparent from the following description of preferred embodimentsthereof.

DESCRIPTION OF THE DRAWING FIG. 1 is a diagram of a typical steppingmotor control arrangement;

FIG. 2 is a schematic of a drive circuit according to the invention;

FIG. 3 is a second drive circuit according to the invention; and

FIG. 4 is a third drive circuit according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates a simplecontrol arrangement for a stepping motor having n phases and therefore ncoils denoted L1, L2, Ln. Other more complicated control arrangements,particularly the so-called 3-2 phase driving sequence, are commonlyresorted to for stepping motor control, and the phase drivers of thepresent invention work equally well under their control. How ever, forsimplicity of description, the control of FIG. 1 is selected toillustrate a typical application of use. Instructions for energizationof the coils are provided by a signal pulse generator 10, which may be acomputer, or other source of programmed pulses, or simply a steady pulsegenerator if the stepping motor is not to have programmed control.Signal pulse generator 10 drives a counter 12 of capacity n. The counter12 has n different outputs 14.1, 14.2, 14!: connected respectively todrive circuits 16.1, 16.2, l6n which supply current to the coils L1through Ln. The counter 12 supplies output 14.1 with the first inputpulse and every nth input pulse thereafter, i.e., l, n 1, 2n 1, etc.Similarly, output 14.2 receives pulse 2, n '+2, 2n 2, etc. Thus counter12 acts both as a frequency divider and as a phase distributor inallocating signal pulses to the coils Ll through Ln. As far as theindividual drive circuit 16.] through 16.1: are concerned, they are torespond only to the individual signal pulses which they receive, andtheir action is thus independent of the action of the other drivecircuits, although they may have common parts such as a common powersupply. In the following description, only one drive circuit andassociated coil will be described even though a plurality of them willtypically be used in a normal multiphase stepping motor.

FIG. 2 illustrates the structure of a generalized drive circuit 16aadapted to supply current IL through a stepping motor coil L for theduration of a signal pulse applied to input terminal 18 from a source 20(which can include a generator and counter 12 as in FIG. 1). Inaccordance with the principles of the invention, drive circuit 16a has asource of energizing potential p in the form of a conventional DC powersupply with alternating current at terminals A,C being rectified byrectifier CR1 and having its energy stored in filter capacitor CF at thepeak AC voltage. Drive circuit 160 has a bridge configuration, andenergizing source p forms one diagonal of the bridge (as FIG. 2 shows,energizing source p can serve other drive circuits for other coils too).The coil L is connected as the other diagonal of the bridge. Diodes D1and D2 are connected to form two opposite sides of the bridge, and areoriented so that they are back biased by energizing source p. Theremaining two sides of the bridge have switch means S1 and 82 connectedtherein, and a switch control means 22, responsive to signal pulses atterminal 18, causes switch means S1 and S2 to change between conductiveand nonconductive states.

During a cycle of operation, switch control means 22 simultaneouslyturns on switch means S1 and S2 and makes them conductive at thebeginning of a signal pulse appearing at input 18. With diodes D1 and D2back biased, the only available path for current to take from source pis through a current path which includes switch S1, coil L and switchS2. As the coil L is energized, its current IL will flow in thedirection shown. When the signal pulse at input 18 terminates, switchcontrol means 22 turns off switch means S1 and S2 and renders themnon-conductive. The stored energy in coil L must discharge with coilcurrent continuing in its original direction. The voltage across coil Ltherefore rises sufficiently to counteract the voltage of source p andforward biases diodes D1 and D2, so that current will flow through apath including diodes D1 and D2 and coil L back into energizing source pwhere it is stored in filter capacitor CF for reuse. When the storedenergy in coil L has been transferred into energizing source p, the coilvoltage drops and diodes D1 and D2 are again back biased and circuit 16ais ready to begin a new coil energizing cycle.

From the foregoing description of the functional operation of diodes D1and D2, it is apparent that they could be replaced by other deviceswhich duplicate their switching function. For example, diodes D1 and D2could be replaced by controlled switches similar to S1 and S2, withmeans similar to switch control means 22 to cause them to change fromconductive to non conductive states with the timing previously describedfor diodes D1 and D2. Other devices could similarly be arranged toperform the diode function of diodes D1 and D2 in the manner described,in the circuits of FIGS. 3 and 4 described below as well as in thecircuit of FIG. 2, and the description of the diode function therein ismeant to include other devices operating in a similar fashion.

Drive circuit 160 is illustrated with a general switch control means 22which controls switches S1 and S2 to cause them to both be conductivewhile coil L is being energized, and to cause them both to benonconductive while coil L is being de-energized. In one conventionalform of stepping motor coil energization, switch control means 22simultaneously actuates switches S1 and S2 to cause them to becomeconductive at the beginning of a signal pulse at terminal 18, maintainsswitches S1 and S2 conductive to apply the direct voltage of source pfor the duration of the signal pulse, and then simultaneously rendersthem nonconductive at the end of the signal pulse. In this conventionalform of stepping motor coil energization, the coil current IL risesexponentially with a time constant determined by the resistive andinductive parameters of the circuit through the coil L. As explainedmore fully in my co-pending application Ser. No. 139,721, filed May 3,1971, this conventional form of stepping motor energization can beimproved upon by energizing the coil with controlled switching of ahigher voltage source, thereby achieving more rapid current rise withefficient use of power sources. One advantageous feature of the presentinvention is that it' is compatible with many coil energizationtechniques, including the energization techniques of the above-mentionedpatent application, illustrated in connection with FIGS. 3 and 4.

FIG. 3 illustrates a stepping motor drive circuit 16b having a bridgeconfiguration with coil L connected across one bridge diagonal.Connected across the other bridge diagonal is an energizing power sourcep of electrical potential high enough to be capable of causing thedesired rapid current increase in coil L. Diodes D1 and D2 are connectedto form two opposite sides of the bridge, and are oriented so as to beback biased by power source p. The remaining two opposite sides of thebridge contain switching transistors Q1 and Q2, oriented so as to beforward biased by power source p. Switch control means 24, similar toswitch control means 22 in FIG. 2, accepts signal pulses at inputterminal 18 from a source 20 and controls the conductivity oftransistors Q1 and Q2, with AND gate 26 and monostable multivibrator 28being provided to turn on transistor Q1 only for a selected intervaltime D sufficient to bring coil current IL to a predetermined peak levelIp. Since the base of'transistor Q1 may be substantially elevated abovethe output of AND gate 26, a level shifting circuit 27 of conventionalconstruction is commonly provided to permit the AND gate 26 output toswitch transistor Q1. The operation of the circuit of FIG. 3 is asfollows: When a signal pulse appears at input 18, transistor Q2 isturned on directly, and transistor Q] is turned on through AND gate 26and multivibrator 28, with the result that power source p is connectedin series with coil L through transistors Q1 and Q2 for the length ofinterval D and coil current IL rises to the predetermined level 1p. Atthe end of interval D, multivibrator 28 turns off and disable AND gate26 to turn transistor Q1 off, thus disconnecting power source p fromcoil L. Coil current IL continues to flow through transistor Q2, whichis still on, and diode D2, now forward biased by the voltage across coilL. Coil current IL will decay according to the resistancecharacteristics of the circuit through transistor Q2, diode D2, and coilL. Particularly at high frequencies, when the duration of a signal pulseat input 18 is-short, current IL does not decay significantly during anenergizing pulse. When the signal pulse at terminal 18 ends, bothtransistors Q1 and Q2 will be turned off. The only available path forcurrent in coil L to take is through diodes D1 and D2, back into powersource p where the energy so carried from the coil L can be stored forreuse in filter capacitor CF. The voltage developed across coil L inorder to discharge through diodes D1 and D2 need only to rise to thelevel at which the diodes become conductive, which is approximately thevoltage of the power source p. Each of transistors Q1 or Q2 thus hasonly this voltage impressed across it during the inductive discharge,which is a lower voltage than that developed in previous devicesdischarging through a Zener diode.

FIG. 4 illustrates another drive circuit 160 employing a bridgeconfiguration, but with a switch control means 30 using a current leveldetecting means 50 to control the connection of the power source p tothe coil L. The bridge configuration of drive circuit 16c connects coilL as one bridge diagonal and the power source p as the other bridgediagonal. Diodes D1 and D2 are connected in two opposite bridge sidesand are oriented so as to be back biased by power source p. In the tworemaining bridge sides, switching transistors Q1 and Q2 are connected,oriented to be forward biased by power source p.

Switch control means 30 has input terminal 18 receiving signal pulsesfrom a source 20 to directly control transistor Q2 and to enable an ANDgate 26 controlling transistor Ql through level shifting circuit 27. Thesecond input to AND gate 26 is from current level detecting means 50arranged to detect the rise of coil current IL to a predetermined peaklevel Ip. Level detecting means 50 has sensing resistor Rs connectedbetween ground and the emitter of transistor Q2 where it receives acurrent comprised of coil current IL and the steady base current ofconducting transistor Q2. A sensing amplifier 52, connected to operateas a triggered switch with input resistor Ri and feedback resistor Rf,has its trigger level established by a biasing battery B3 to correspondto passage through resistor Rs of predetermined current level Ip plusthe base current of transistor Q2. A suitable example for sensingamplifier 52 is Fairchild Model 710 dual in line I.C., with connectionsmade to pin terminals identified in FIG. 4 as f.g. "P3) for pin numberthree. Level detecting means 50 has an output to AND gate 26 whichenables AND gate 26 and turns on transistor Q1 at the beginning of asignal pulse at input 18 (when no current yet flows through resistorRs), and disables AND gate 26 and turns off transistor Q1 when currentrises to predetermined level Ip. In addition, level detecting means 50is arranged to again enable AND gate 26 and turn on LII transistor Q1 ifcoil current falls from predetermined level lp to a lower switchinglevel Is as set by feedback resistor Rf of amplifier 52. Coil current ILcan thus be carefully controlled to remain within the range Ip to Iswithout using a secondary potential source to maintain current andwithout relying on slow decay characteristics or high frequencies toprevent coil current from falling significantly.

Operation of drive circuit 160 is as follows:

Upon arrival of a signal pulse at input terminal 18, transistor O2 isturned on directly and transistor Q1 is turned on through AND gate 26,both inputs of which are then present. Power source p energizes coil Lthrough a circuit path including transistors Q1 and Q2 and resistor Rs.When coil current IL rises to predetermined level Ip, as sensed byresistor Rs, .level detecting means 50 disables AND gate 26 andtransistor O1 is turned off. Coil current IL continues to flow, througha second circuit path including transistor Q2, resistor Rs, and diodeD2, until it falls to switching level Is as sensed by resistor Rs. Leveldetecting means 50 thereupon enables AND gate 26, reconnecting powersource 6 p through coil L to raise current again to level Ip. Coilcurrent IL continues to oscillate between the level Ip and ls for aslong as a signal pulse appears at input 18.

When the signal pulse ends, transistors 01 and 02 are turned off and thecollapsing field in coil L sends current through a thirdcircuit pathwhich includes diodes D1 and D2 back into capacitor CF of power source pfor storageaAs in the case of drive circuit 16b, the maximum voltageimpressed across either transistor Q1 or O2 is limited to the voltageacross capacitor CF.

The general drive circuit illustrated in FIG. 2 can be modified in otherways for compatibility with other coil energization techniques as willnow be apparent. For example, instead of using rectified DC power sourcep to energize coil L, a storage battery could be used instead, or adifferent power source whose potential was varied with time forparticular purposes could be employed. With any such modification, thedrive circuitaccording to the invention still provides a dis chargingcurrent path back into the power source to permit the coil energy to berapidly transferred from the coil in a way enabling it to be stored forsubsequent reuse.

It should be understood that the present disclosure is for the purposeof illustration and that the invention includes all modifications andequivalents falling within the'scope of the appended claims.

I claim:

1. A stepping motor drive circuit for supplying current to' a coil forthe duration of a signal pulse comprising, in bridge configuration:

a source of energizing power connected as one bridge diagonal;

the coil being connected as the other bridge diagonal;

diode means connected in each of two opposite bridge sides and beingback biased by said energizing power source;

controllable switch means in each of the remaining two opposite bridgesides; and

control means for said switch means, responsive to a signal pulse, forturning on said switchmeans simultaneously to provide a first currentpath through said switch means for energizing of the coil, for turningoff one of said switch means after a preselected time interval, saidcoil remaining energized after said time interval, and for turning offthe other of said switch means at the end of said signal pulse toprovide a second current path through said diode means for discharge ofstored coil energy back into the energizing power source.

2. A stepping motor drive circuit according to claim 1 wherein saidswitch means comprise transistors forward biased by said energizingpower source; and

said control means includes a monostable multivibrator operative forsaid preselected time interval.

3. A stepping motor drive circuit according to claim 1 wherein saidenergizing power source includes a capacitive storage element andwherein energy is transferred from said coil to said element throughsaid diode means.

4. A stepping motor drive circuit for supplying current to a coil forthe duration of a signal pulse, comprising:

an energizing power source;

first circuit means providing a first current path through said powersource and coil thereby to cause current to flow in said coil;

second circuit means providing a second current path through said coilafter a preselected time interval beginning of a signal pulse, therebyto energize the coil, for connecting the coil through the second circuitpath after said preselected time interval, thereby to maintain the coilenergized, and for connecting the energizing power source and coilthrough the third circuit path at the end of said signal pulse, therebyto discharge the coil energy back into the power source.

t t t t

1. A stepping motor drive circuit for supplying current to a coil forthe duration of a signal pulse comprising, in bridge configuration: asource of energizing power connected as one bridge diagonal; the coilbeing connected as the other bridge diagonal; diode means connected ineach of two opposite bridge sides and being back biased by saidenergizing power source; controllable switch means in each of theremaining two opposite bridge sides; and control means for said switchmeans, responsive to a signal pulse, for turning on said switch meanssimultaneously to provide a first current path through said switch meansfor energizing of the coil, for turning off one of said switch meansafter a preselected time interval, said coil remaining energized aftersaid time interval, and for turning off the other of said switch meansat the end of said signal pulse to provide a second current path throughsaid diode means for discharge of stored coil energy back into theenergizing power source.
 2. A stepping motor drive circuit according toclaim 1 wherein said switch means comprise transistors forward biased bysaid energizing power source; and said control means includes amonostable multivibrator operative for said preselected time interval.3. A stepping motor drive circuit according to claim 1 wherein saidenergizing power source includes a capacitive storage element andwherein energy is transferred from said coil to said element throughsaid diode means.
 4. A stepping motor drive circuit for supplyingcurrent to a coil for the duration of a signal pulse, comprising: anenergizing power source; first circuit means providing a first currentpath through said power source and coil thereby to cause current to flowin said coil; second circuit means providing a second current paththrough said coil after a preselected time interval such that current ismaintained in said coil after said time interval; third circuit meansproviding a third current path through said power source and coil suchthat current induced in the coil through the first and second circuitpaths is directed in the third path back into the energizing powersource; and control means for connecting the energizing power source andcoil through the first circuit path at the beginning of a signal pulse,thereby to energize the coil, for connecting the coil through the secondcircuit path after said preselected time interval, thereby to maintainthe coil energized, and for connecting the energizing power source andcoil through the third circuit path at the end of said signal pulse,thereby to discharge the coil energy back into the power souRce.